This application claims the priority of German Patent Application Serial No. 199 48 830.4, filed Oct. 6, 1999.
The present invention relates, in general, to a telescopic boom for cranes, especially mobile cranes.
German Pat. No. DE 197 11 975 A1 describes a telescopic boom of a type including a main boom having a base section and a plurality of boom sections received in the main boom and telescopically extendible and retractable. The base section and the boom sections are each made of a profile having an upper shell and a lower shell which are welded together at their confronting walls. The upper shell has a nominal wall thickness which is typically smaller than a nominal wall thickness of the lower shell, although there are other designs which propose to fabricate the upper and lower shells of same nominal wall thickness.
In the description, the term xe2x80x9cnominal wall thicknessxe2x80x9d has been selected to make clear that fluctuations of the wall thickness in transverse and longitudinal directions during fabrication should be disregarded. The nominal wall thickness of both shells should thus be considered constant in transverse and longitudinal directions, whereby the zone that is subject to greatest stress determines the magnitude of the wall thickness. The way the wall thickness for this zone of greatest stress is dimensioned establishes also to a substantial degree the weight of the boom. A further parameter is the desired length of the boom by which the required number of telescoping boom sections is determined. Road traffic regulations impose a maximum permissible axle load so that as a result of the weight of the boom the limits of the load-carrying capability and hoisting height are very quickly reached. Dismantling of the entire boom and separate transport of the dismantled boom may be one of the options to solve the stated problem. Assembly and disassembly of the boom is, however, complex and labor-intensive and can be further compounded by conditions at the job site. Moreover, it requires provisions for an additional transport.
It is thus an object of the present invention to provide an improved telescopic boom for cranes, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved telescopic boom for cranes which permits a higher load-carrying capability at same limiting factors for the hoisting height and maximum permissible axle load.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing a telescopic boom having a base section and a plurality of boom sections received in the base section and telescopically extendible and retractable, wherein the base section and the boom sections are each made of a profile comprised of an upper shell and a lower shell which are joined together at their confronting walls, with the upper shell having a nominal wall thickness which is equal or smaller than a nominal wall thickness of the lower shell, wherein at least one portion of one of the upper and lower shells includes, as viewed in cross section, two outer thin metal sheets placed in spaced-apart relationship to define an intermediate space therebetween.
Suitably, the intermediate space may be filled, partially or entirely, with filler material. For strength and weight reasons, an appropriate filler material is metal foam, preferably aluminum metal foam. Such a sandwich structure has the advantage that the own weight of the boom can be drastically reduced, without compromising the stiffness and buckling strength. A somewhat more complex fabrication of the boom is more than made up by the increased load-carrying capability. The advantages of the sandwich structure can be further enhanced, when the shell has in transverse direction a nominal wall thickness that is commensurate with the load. In this way, those zones that are subject to greatest stress can be designed with the required wall thickness whereas zones that are subject to less stress may have a thinner wall thickness.
According to another feature of the present invention, areas of the sandwich structure that are exposed to great local force introduction, e.g. bolting area, should be provided with a reinforcement. For example, the intermediate space between the metal sheets may be filled with solid material, or outer metal sheets may have a greater wall thickness.
Of course, the general concept described herein for making the base section and the telescoping boom sections of sandwich structure, is equally applicable to other elements of the boom. For example, box-like extensions which may be mounted to the boom head of the fly section can also be fabricated by a sandwich structure according to the present invention.
The weight distribution in the head area of booms is of particular relevance and, to a large extent, governs the exploitable load-carrying capability. In this context, it should be noted that the application of the sandwich structure is independent of the cross sectional configuration of the boom sections, i.e. it is secondary whether the boom section is rectangular, ovaloid, oval or round. The concepts of the sandwich structure is also applicable for latticed tower cranes. Critical hereby are the chords that are subject to buckling loads. A filling of the chords with metal foam results in a significant reduction of the wall thickness of the chords and thus in a weight reduction.